Is the prayer of the congregants valid, and is it permissible for a shaykh who prays sitting due to his inability to lead the prayer, while the congregants pray standing?

The prayer of the congregants behind a sitting Imam is valid, though it is preferable that someone who is unable to stand does not lead them. If the Imam begins his prayer sitting, the congregants must also pray sitting, and this is the most preponderant opinion. As for the hadith of Abu Bakr, it is responded to by stating that Abu Bakr began the prayer standing, then the Prophet, peace and blessings be upon him, sat down. This means that the congregants continue their prayer standing if something happens to the Imam that causes him to sit during the prayer. Prayer behind an Imam who is unable to prostrate is valid, according to the preponderant opinion. What has passed of the congregants' prayer standing behind a sitting Imam is considered valid due to their ignorance of the ruling, but after gaining knowledge, they must pray sitting.